Oxidation Mechanisms in Zircaloy-2 - The Effect of SPP Size Distribution

Transcription

1 Oxidation Mechanisms in Zircaloy-2 - The Effect of SPP Size Distribution Pia Tejland 1,2, Hans-Olof Andrén 2, Gustav Sundell 2, Mattias Thuvander 2, Bertil Josefsson 3, Lars Hallstadius 4, Maria Ivermark 4 and Mats Dahlbäck 4 1 Now with Studsvik Nuclear AB, Nyköping, Sweden 2 Chalmers University of Technology, Göteborg, Sweden 3 Vattenfall Nuclear Fuel AB, Stockholm, Sweden 4 Westinghouse Electric Sweden, Västerås, Sweden

2 Outline Introduction Materials Oxidation behavior Questions Microstructure Oxide Metal Oxygen ingress Transition mechanism Conclusions

3 Material A Material B Materials Zircaloy-2 SPPs: Zr(Fe,Cr) 2 and Zr 2 (Fe,Ni) Two commercial Zircaloy-2 alloys with different heat treatments: - Material A has more but smaller (22 nm) SPPs - Material B has fewer but larger (84 nm) SPPs Oxidized in static steam autoclave to obtain different oxide thicknesses. Chemical composition Sn (%) Fe (%) Cr (%) Ni (%) Si (ppm) O (ppm) N(ppm) SPP size Average SPP diameter (nm) Mechanical properties Yield stress (MPa) Fracture stress (MPa)

4 Weight gain (mg/dm2) Weight gain (mg/dm2) Oxidation behavior in autoclave A 400 C A 415 C B Material A Material B Zirlo TM 50 B Material A Material B Zirlo TM Time(days) Time(days) The results from autoclave corrosion testing show that Material B has a better corrosion resistance than Material A.

5 Oxidation behavior in reactor Material A Material B Material B shows better corrosion resistance also in reactor.

6 Questions How can the oxidation process in Zircaloy-2 be understood in terms of microstructure and oxygen ingress in the metal/oxide interface region? Can any mechanisms be found explaining the relation between SPP size distribution and corrosion rate? Method: The metal/oxide interface region has been investigated using transmission electron microscopy (TEM) and atom probe tomography (APT).

7 Metal/oxide interface morphology (1) (BF TEM image) Initial observations: 1. The interface is undulating/wavy. 2. There are lateral cracks in the oxide scale.

8 Metal/oxide interface morphology (2) (BF TEM images) Columnar grains, ~20 30 x 200 nm. Inward diffusion of oxygen faster in oxide grain boundaries than in bulk.

9 The undulating interface The metal/oxide interface is undulating on a micrometer scale. No obvious difference in period or amplitude was found between materials A and B. 1 µm 2 µm 9 µm [Tejland et al. J. ASTM Int. 8 (2011)] A cyclic dependence on oxide thickness was found. [Tejland et al. J. Nucl. Mat. 430 (2012)] Parise et al. have shown by modeling that variations in diffusion rate in different oxide grain boundaries will give rise to the characteristic wavy interface. [Parise et al. J. Phys. IV France 9 (1999)]

10 Volume expansion upon oxidation The transformation from Zr to ZrO 2 has a volume expansion of 55 % (Pilling-Bedworth ratio = 1.55), with 54% expansion in radial, 0.5% in axial and 0.5% in tangential direction. [Parise et al. 1998] t or a oxide metal r This volume expansion causes stresses to be built up in the material as oxidation proceeds.

11 Undulations + volume expansion => stresses a oxide A B C D - Volume A-B is oxidized and expands 54% until C. At D no oxidation has occurred. - For a stress free material, voids would have to form under wave crests. metal However, no voids are created, the interface does not break. b oxide metal Re-drawn after Parise et al. J. Nucl. Mat. (1998) - Instead, large tensile stresses are created at wave crests. - At wave valleys, compressive stresses are created. - Parallel to the interface, compressive stresses exist in the oxide and tensile stresses in the metal. The stresses in the oxide lead to crack formation in the metal lead to creep deformation

12 Oxide: Crack formation The tensile stress in the oxide is a driving force for crack formation, but also an initiation site is needed. The following crack initiation is proposed: Intermetallic second phase particles (SPPs) oxidize slower than the surrounding matrix => absent volume increase adjacent to SPPs. The tensile stress above the particle can be relieved in two ways: i) By forming a void above the particle. ii) By forming a crack further up in the oxide. [Tejland et al. J. ASTM Int. 8 (2011)]

13 Oxide: Voids and cracks BF TEM images of voids adjacent to SPPs. There are more lateral cracks in Material A (with more SPPs) than in Material B (with fewer SPPs).

14 Metal: Microstructure underneath the oxide scale Evidence of plastic deformation: Areas with high dislocation density Oxide Metal Small hydrides

15 Metal: Small hydrides HAADF BF - Needle shaped streaks, most clearly visible using HAADF contrast but also using BF STEM. - Shown by EELS to be hydrides (shift in the plasmon peak of about 2 ev). However, it is important to notice that these hydrides are formed during cooling from autoclave temperature, and therefore not present during the oxidation process!

16 Metal: Creep deformation Oxide Metal Evidence of heavy plastic deformation: - Areas of high dislocation density - Twinning - Sub-grain boundary formation Areas of high dislocation density A result of the high stress levels caused by oxidation. No apparent difference between Material A and B. Twin lamellae Sub-grain boundaries

17 Sub-grain boundaries Decorated with Fe and Ni Inherited by the oxide APT reconstruction Oxide Metal Sub-grain boundary Iron Nickel

18 Schematic mechanism These decorated sub-grain boundaries are important for hydrogen transport, but might also play a role in oxygen diffusion. 5 6 [Sundell, G. et al., Corr. Sci. 65 (2012)]

19 Microstructure Summary Correlation to SPP size distribution SPPs are initiation sites for crack formation => more SPPs lead to more cracking SPPs increase the strength of the material through particle hardening => less stress relaxation by plastic deformation => more stresses remaining in the material => more cracking The transition in corrosion kinetics is connected to this cracking of the oxide scale Conclusion: Material B, with fewer (and larger) SPPs should exhibit the superior corrosion resistance.

20 Oxygen ingress Not a sharp step from the 67 at. % oxygen of ZrO 2 to 1 at. % in the metal. Using EDX in TEM, a suboxide with ~50 at. % (ZrO) was found, and in some cases also a step to ~30 at. %. The oxide is pre-transition, 1 µm thick.

21 >60 at.% ZrO 2 O sat. Zr <32 at.% From >60 to <32 at. % within 20 nm Two 2D slices from the same APT specimen Varying suboxide layer thickness Oxygen ingress APT heat maps, red is max concentration and blue is min. All profiles from 1 µm thick oxide scales Varying thickness and morphology of the ~50 at.% oxygen suboxide layer GB ~1 < >60 at. % Always a step to ~31 at.% oxygen. Higher than the oxygen solubility limit (28.6 at.%), an effect of deformation? Crystallographic dependence of oxygen diffusion in metal.

22 Oxygen ingress Fe ZrO 2 Presence of grain boundary demonstrated by Fe segregation.

23 ~60 at. % Oxygen ingress ~60 at. % at. % at. % <30 at. % <30 at. % nm nm Uneven interface also on this scale Oxygen ingress appears to be crystallographic Enhanced O ingress below the grain boundary Fe Ni ZrO 2 Blue areas contain >45% O

24 Oxygen ingress The ZrO suboxide layer sometimes consists of a mixture of regions with different oxygen content. Fingers of high oxygen content (~60 at. %) with a diameter of approximately 5 nm penetrate ~50 nm into regions of lower oxygen content (~50 at. %). 2D slice (nm)

25 Oxygen ingress Red color: Metal with over 50 at. % zirconium. Blue color: Oxide with over 56 at. % oxygen.

26 Oxygen ingress Summary Investigation of 1 µm, pre-transition oxide scales shows a large variation in oxygen ingress into the metal. A suboxide layer containing ~50 at. % oxygen exists in some cases, with varying thickness and morphology. This finding suggests that the kinetic transition in this material might be a localized phenomenon. Always a step to ~31 at. % oxygen followed by a diffusion profile of varying width. 31 > 28.6 at. %, which is the solubility limit for oxygen in zirconium. Due to the heavy plastic deformation? No obvious difference in oxygen ingress between Material A and Material B.

27 Transition mechanism Zircaloy-2 does not exhibit the same cyclic oxidation behavior as other zirconium alloys. The material might still go through a transition but it seems to be more of a local phenomenon rather than an overall sudden change along the metal/oxide interface. APT investigations show varying oxygen gradients into the metal in samples with the same oxide thickness, pretransition Transition is assisted by a weaker oxide scale and a harder metal matrix, as that kind of material will have an oxide that is easier to crack. This explains why a material with more but smaller SPPs will have a poorer corrosion resistance than one with fewer but larger SPPs.

28 Conclusions The transition in oxidation rate in Zircaloy-2 is a local phenomenon. A material with many small SPPs will have a weaker oxide scale and a harder metal matrix, both properties leading to a poorer corrosion resistance. Thank you!